Bi-material condition sensing means

ABSTRACT

A bi-material temperature and/or humidity sensor including a circular disc which is mounted in fixed relation about the periphery. A flexible metal base member has a second material bonded to the center or to the outer portion to form a bimaterial element. The second material is made of a substantially smaller area.

Unite States Patent Dube et al.

[54] BI-MATERIAL CONDITION SENSING MEANS Inventors: Victor J. Dube, WestAllis; Paul E. Thoma, Milwaukee; Jao Shiun Kao, Wauwatosa,

all of Wis.

Assignee: Johnson Service Company, Milwaukee,

Wis.

Filed: Apr. 11, 1968 Appl. No.: 720,684

US. Cl ..73/337, 73/363 Int. Cl ..G0lm 19/10, GOlk 5/62 Field of Search..73/337, 363.3, 363.1, 363.5,

[ 5] May 30, 1972 56] References Cited UNITED STATES PATENTS 3,461,7239/1969 Thoma ..73/337 1,822,069 9/1931 Thomas ..73/378.3

Primary ExaminerLouis R. Prince Assistant Examiner-Denis E. CorrAtt0rneyAndrus, Sceales, Starke and Sawall 5 7] ABSTRACT 23 Claims, 8Drawing Figures Patented May 30, 1972 RADII RATIO-z BI-MATERIALCONDITION SENSING MEANS This invention relates to bi-material conditionsensing means for sensing and/or controlling any condition orenvironment which effects a different dimensional change in one materialwith respect to another and particularly for sensing and/or controllinghumidity, temperature and the like.

Bi-material sensors have been widely employed in the environmentalcontrol field to sense and to control humidity and temperatureconditions. Generally, bi-material sensors have employed two differentmaterials bonded in superimposed relation with the materials having adifferent coefficient of expansion and contraction with respect to thesensed condition. Generally, the materials are selected to include aflexible base member which is essentially insensitiveto the conditionand a second material which is substantially sensitive to the condition.As a result, any change in the condition results in a differentialmovement of the two materials with a resultant deflection of thebi-material sensor. The more common variety employs a cantilevered beamor plate having a bimaterial deflection sensing unit secured to orforming a part of the free end thereof. Although such systems provide asatisfactory means for sensing of temperature and/or humidity, theelemental associated components are subject to corrosion and collectionof foreign particles from the surrounding atmosphere. There is also asubstantial need fora sensor which can define an enclosure. Further, thepresent construction provides for varying of the deflection only by achange in material and/or a change in the relative thickness of the twolayers of the two materials.

A bistable sensor producing a snap action type control employs acircular plate or shell which is simply supported to allow relativemovement with respect to a support means such that actual deflectionresults in response to changes in a sensed condition. The constructionof a suitable support for the bi-material disc type units has presenteddifficulties which have so limited the application such that simplysupported bimaterial discs are generally employed in bistable, snapaction temperature controls and not in proportional controls or inhumidity controls.

Thus, the disc type element cannot be rigidly mounted about the edgewithout complete loss of deflection. A shelltype construction having acup-shaped member with the base portion covered with a second materialhas also been tried. The unit is similar to a rigid mounting and verylittle deflection is detected.

Although various suitable bi-materials have been known and employed formany years for sensing temperature and humidity conditions, US. Pat. No.3,301,057 and the copending application of Paul E. Thoma entitledSynthetic Humidity Sensing Element and Method of Preparing the Same,which was filed on July 10, 1967 with Ser. No. 652,287, now US. Pat. No.3,461,723 disclose particularly satisfactory humidity sensitivematerials of a synthetic composition for detection of humidityconditions. The materials have physical strength and more linearhumidity coefficients of dimensional change with humidity which makesthem particularly suitable for proportional controls.

The present invention is particularly directed to an improvedbi-material sensing device particularly of the disc type includingintegral means to control the deflection sensitivity of the unit. Thesensitivity of the unit is defined as the per unit deflection per unitchange in the sensed condition such as the relative humidity ortemperature or other condition which causes the material to expand andcontract.

Generally, in accordance with the present invention, the sensing deviceis a disc or wafer element including a generally flat or substantiallyplanar base member having a selected coefficient of expansion andforming a support for the sensor element. The member is provided with afixed mounting at the periphery. The other or second material is appliedand bonded to the base member in any suitable manner to form abimaterial element, but is made of a substantially smaller configurationto define a free portion or area of the first material or element. Thefree portion or area in accordance with an important aspect of thepresent invention is symmetrically arranged with respect to the supportand the second material. Applicants have found that with this newconstruction, the peripheral portion may be essentially fixed and anappreciable deflection of the bi-material element not connected to themounting means obtained. This differential area construction thusestablishes a simple mounting for a disc type sensor and permitscomplete enclosure of one side of a proportional sensing element fromthe environment in which the sensor is mounted.

The bi-material element may take a variety of different preferredconfigurations of overlying bonded materials employing a base member towhich an outer ring-shaped and/or a central disc-shaped second materialis bonded.

As applied to a preferred circular member, the second material isapplied to the mounting member as a central disc having a substantiallysmaller radius than the mounting member or as a ring member with asubstantial central opening. The bi-material element is essentiallyfixedly mounted about the periphery to prevent movement thereof.

Applicants have found that the sensitivity of the device is related toand controlled by the relative areas or radii of the two materials andthat an optimum ratio exists. Further, the optimum ratio can, at leastfor a circular disc for sensing temperature and/or humidity, be readilydefined in a mathematical equation. Generally, it has been found thatthe sensitivity increases to a maximum as the relative area and radiusof the applied or second material is decreased to a selected ratio butthat continued reduction results in a decreasing sensitivity.

Alternatively, the sensor may also be constructed in accordance with thepresent invention with materials having different coefficients ofexpansion defining an inner portion of a disc and the same or similar,but oppositely disposed, materials defining the outer portion of thedisc.

The present invention provides a very simple and reliable means ofproviding a fixed type sensor which can be readily mass produced withcontrolled sensitivity. The simplified mounting permitted by the presentinvention thus provides a highly satisfactory deflection sensing devicewhich can be readily placed within an enclosed environment.

Further, a cross-linked material such as disclosed in the previouslyidentified copending application of Paul E. Thoma produces an unusuallysatisfactory and operative humidity sensor of the disc type with theouter edge fixed. The characteristics of the cross-linked material andparticularly the moisture sensitivity, the bonding quality andresistance to chemical attack produce an unusually satisfactorydeflection of the disc element.

The drawing furnished herewith illustrates preferred constructions ofthe present invention in which the above advantages and features areclearly disclosed, as well as others which will be readily understoodfrom the following description.

In the drawing:

FIG. 1 is a pictorial view of a cylindrical shell bi-material sensorconstructed in accordance with the present invention;

FIG. 2 is a top view of the sensor shown in FIG. 1;

FIG. 3 is a vertical section taken on line 33 of FIG. 2;

FIG. 4 is a graphical illustration showing the sensitivity of a unitconstructed as shown in FIGS. 1-3 and 5 with varying radii of one of thebi-material elements or materials;

FIG. 5 is a vertical section of a rigidly mounted sensor element;

FIG, 6 is a view similar to FIG. 5 showing a further construction;

FIG. 7 is a view similar to FIG. 6 showing another construction inaccordance with the present invention; and

FIG. 8 is a view similar to FIG. 5 showing an alternative constructionof the sensor element.

Referring to the drawing and particularly to FIGS. 1-3, a bimaterialsensor which by proper selection of materials can be employed to sense atemperature, humidity or other condition is shown constructed inaccordance with the present invention. The present invention may whencombined with the humidity sensing material of the previously identifiedapplication provide an unusually satisfactory humidity sensing apparatusand the illustrated embodiments are described in connection with such anovel humidity sensor. The illustrated sensor includes a cylindricalsupporting shell 1 defining a generally cup-shaped member having amounting flange 2 integral with the peripheral edge thereof. Theillustrated flange 2 is apertured for bolting or otherwise securing ofthe sensor to a sup port 3 within a suitable environment. Thecylindrical shell I is designed with a base 4 of a selected radius andof a flexible material having a selected coefi'icient of humidityexpansion with respect to the humidity condition to be sensed.

A second material layer or coating 5, of a substantially differentcoefficient of expansion than that of the base 4, is applied to theouter face of the base 4 in the illustrated embodiment of the inventionto define a bi-material sensor. The element 5 may be a completelyseparate fabricated member suitably bonded throughout its interface tothe base of shell 1 or may be a deposited layer or coating. For example,the

. previously identified patent and copending application of Paul E.Thoma disclose satisfactory synthetic materials for defining a varyingdimension humidity sensing element. Generally, the materials noted inthe patent provide satisfactory synthetic humidity sensitive materialsincluding an organic polymer being substantially free of hydroxyl,carboxyl, amino and imino groups. As more fully developed in thecopending application,

an exceptionally satisfactory material for layer 5 is one attached bycross-linking such as cellulose acetate butyrate (37 percent combinedbutyryl) which is cross-linked with ureaformaldehyde. Applicants haveapplied such material to a shell of aluminum (alloy llO-O) which wasfirst zinc plated and then copper plated by flash coatings. The surfaceof the copper may be oxidized to further improve the adhesion of thecross-linked cellulose acetate butyrate layer 5 to the base 4.

The coefficient of humidity expansion of the above cellulose acetatebutyrate is such that the material will generally show a dimensionalincrease greater than I percent with a change in relative humidity fromto 100 percent while the aluminum shows essentially no change because ofa 0 humidity coefficient. The synthetic material is particularlysignificant in the present invention as applied to a humidity sensorbecause of its sensitivity characteristic and because it can be appliedin an accurate and controlled manner with an excellent bond to a metalbase material or member.

Although the significant factor in the present invention as in allbi-material sensors is the relative coefficients with respect to theconditions being sensed, a flexible base member having a zero or verylow coefficient is nonnally selected in the practical construction.

In accordance with the illustrated embodiment of the invention, theapplied second material layer is formed with a substantially smallerradius than the base 4 and preferably equal to essentially 65 percent ofthe radius of the base for the following reasons.

Applicants have found that the base 4 with the layer 5 defines a discunit, the sensitivity of which varies with the relative radii of thebi-material layer 5 and the base 4. In the particular construction, thedeflection characteristic essentially conforms to the characteristicsshown in FIG. 4. In the characteristics of FIG. 4, the sensitivity isshown on the vertical axis in inches per percent of humidity change andthe relative radii is shown on the horizontal axis as the ratio ofradius of the layer 5 to the radius of the base 4 of the shell.Generally, as the radius of layer 5 increases from zero, the deflectionsensitivity also increases until at approximately 0.65 radii ratio, amaximum is reached after which the sensitivity decreases. \Vith thebi-material layer 5 corresponding in diameter to that of the base 4, avery minute and essentially zero deflection sensitivity results.

Applicants have found that the sensitivity characteristic of a shelltype unit as shown in FIGS. 1-3 is essentially dependent NOMENCLATURE a=Radius of layer 5 b= Radius of layer or base 4 E, Modulus of elasticityfor material a B, Modulus of elasticity for material b h Thickness ofmaterial a h Thickness of material b a Expansion coefficient due totemperature for material 0 Expansion coefficient due to temperature formaterial B Expansion coefficient due to humidity for material a B,Expansion coefficient due to humidity for material b v Poisson's ratio(assumed to be equal for both materials S Deflection at the center AT=Change in temperature ARH=-Change in relative humidity W= Deflection atthe center per unit change of stimulus (relative humidity ortemperature) Furthermore, applicants have derived the following equationwhich defines the sensitivity characteristic of a cylindrical shellhumidity sensor, as shown in FIG. 4 in terms of said variable with thetemperature coefficients of both materials (oz, and a equal to 0 or toeach other or the temperature condition held constant and the humiditycoefficient of the base (5,) is equal to essentially O:

where:

50 b 0 64; (1 V) in The same equation is equally applicable to atemperature sensor wherein the humidity coefficients are equal to zeroor to each other or the humidity is held constant and the temperaturecoefficient of the base (ca is essentially zero. The term N, howeverwould be replaced with the term N,,, (E h,, n

In an actual construction similar to that shown inFlG. 1, actualexperimental results for a number of difierent radii are shown in FIG. 4by the several circled dots 7 and are seen to very closely agree withthe characteristic curve 6 as defined by the mathematical equation. Thebase metal and the applied element were as previously identified and hadthe following material constants and dimensions:

B 2 X 10" in/in/% RH h,,= 8.4 X 10 in.and

b= l in. 1

The experimental result discloses a similar maximum sensitivity atrelative radii ratio of 0.65 for the humidity sensor constructed of thepreviously identified materials and similar materials. As the materialconstants and the dimensions control the final characteristic generally,however, no definite optimum ratio for all designs of the sensors can beset forth. However, such determination can be readily obtained by anysimple test procedure, solving of the analytical equation with acomputer or the like.

The shell I mounted on the support 3 defines an enclosed chamber 8within which components can be secured and {no tected from thesurrounding environmental dirt, moisture and the like. Thus, in thedrawing, an output element 9 is diagrammatically shown connected to thesubstrate or base 4.

An alternative construction is shown in FIG. 5 wherein a base orsubstrate plate 10 of circular configuration is rigidly fixed about itsperiphery in a mounting fixture or wall 11. The plate 10 corresponds tothe base 4 of the shell 1 of the first embodiment. The second materialor layer 12 of the bi-material structure is bonded or otherwise securedto the central portion of the plate and is provided with a selectedsmaller radius to produce the desired deflection characteristic.

The applicants have devised a similar equation for the total deflectionof such an element in response to both temperature 1 and humidityconditions,'as follows:

: a aBa b bBb u AT NH! ARH Enhgaa-Ebhgab ISOI WH MII GO] c wfi andwherein B C,,, D B C and D,, are as previously defined.

The above equation may be reduced to define a humidity sensitive elementfor comparison with the characteristic of the shell unit by setting ofAT to and the humidity coefficient of the substrate or base to O.Plotting of the modified equation defines a similar characteristic,shown by curve 13 in FIG. 4 which essentially corresponds to thecharacteristic curve 6 for the shell construction. Curve 1,3 is shown indashed line and superimposed on the characteristic curve 6 of the shellunit in FIG. 4 for clearly showing the effect of the rigid mounting. Thelatter characteristic curve 13 was obtained employing the same materialconstants and element dimensions. The fixed unit of FIG. exhibits asimilar maximum sensitivity at about 0.65 radii ratio although the totaldeflection was somewhat less. The deflection sensitivity however is 0when the layer 12 and base or substrate layer are coextensive. This isin contrast to the shell unit where the coextensive layer constructionprovides some minute deflection. It appears from applicants analysis ofthe new structure that, with coextensive layers which are fixed orrigidly mounted about the periphery, the deflection of the disc in thecentral portion is opposed by the outer portion such that there isessentially no deflection. The slight deflection of the shell unit maybe explained by the slight give provided by the cylindrical side wall.

The deflection of the embodiment shown in FIG. 8 may be found from thefollowing equation, also derived by the applicants, for any twomaterials employed in the structure of FIG. 8, wherein a refers to theradius of the central materials 23 and 25 and b refers to the radius ofthe element to the mounting:

III

where N N M M B C, and D are the same as previously set forth and u i iiu a l b 211 111( In FIG. 6, a base member 14 is rigidly fixed within asupport wall 15 similar to the mountings of FIG. 5. An annular secondand moisture sensitive layer 16 is applied to the outer peripheralportion of member 14 and includes a substantial. opening 17 defining afree area on member 14. The wafer or disc bi-material element of FIG. 6will respond to an increasing humidity condition to cause a downwarddeflection of the element and the deflection characteristic willgenerally be that previously described and will be determined byrelative radii of the base member 14 and the opening 17 which alsodefines the radial length or area of the material layer 16.

A modified embodiment of the condition sensor is shown in FIG. 7 whereinan annular condition sensitive layer 18 is applied to the bottom side ofa base element 19. A disc-shaped layer is applied to the top and centerof the base member 19 and the wafer element is secured with a wall 21.If the base member 19 is formed of a material with a small or O humiditycoefficient and layers 18 and 20 with a relatively high humiditycoefficient, in accordance with the previous discussions of the drawing,the element will deflect upwardly, generally in accordance with theprevious discussions of the actions of the other embodiments. Y

A further embodiment which in a thin layer element such as previouslygiven approximating the embodiment of FIG. 7 is shown in FIG. 8including a disc element similarly secured within a wall 22. In FIG. 8,the top and bottom coextensive but dissimilar multiple layers areintimately bonded to each other. The top layer includes a centralcircular disc portion 23 integrally formed within an outer encirclingportion 24, the periphery of the latter being embedded within the wall22. The bottom layer similarly-includes a central circular disc portion25 with an interconnected outer encircling portion 26. As illustrated bysimilar cross-sectioning, the upper central disc portion 23 and thelower outer portion 26 are of the same or similar material having afirst coefficient while the lower central disc portion 25 and the upperencircling outer portion 24 are of the same or similar material having acoefficient different than the first coefficient. v

Although a particular circular shell and plate disc construction havebeen illustrated, any other type of mounting for the units can beprovided. Although the element sensitivity can be controlled by varyingthe several parameters noted at page 8, a significant aspect or featureof the present invention is ease of control afforded by controlling therelative radii of two materials in the form of disc elements essentiallyrigidly held.

We claim:

1. In a bi-material condition sensor adapted to establish a deflectionin accordance with the sensed condition comprismg,

a flexible substantially planar base member of a material having apredetermined coefiicient of expansion with respect to said condition,

a second material having a selected different coefficient of expansionand being bonded at the interface to a selected portion of said basemember, said selected portion being substantially less than the area ofsaid base member to define a substantially planar bi-material elementwith said base member having a substantially uncovered portion, saidbi-material element deflecting normal to said element, and

fixed and rigid mounting means connected at opposite sides of said basemember to said bi-rnaterial element to rigidly support said element andestablish movement of only the portion of the element not connected tothe mounting means with the movement of said element in a directionnormal to the plane through said fixed mounting means.

2. The bi-material condition sensor of claim 1 wherein said fixedmounting means is connected essentially to the full periphery of saidbase'member.

3. The condition sensor of claim 1 wherein said second material issymmetrically secured to the base member.

4. The bi-material condition sensor of claim 1 wherein said secondmaterial is integrally bonded throughout the interface to the centralportion of said base-member in substantially spaced relation to saidfixed mounting means.

5. A'bi-material sensor in accordance with claim 4 wherein the basemember is rigidly held about its periphery.

6. The condition sensor of claim 1 wherein said base member is acircular plate and said mounting means is secured to the peripherythereof and said second material is secured to the center of the basemember.

7. In a bi-material humidity and temperature sensor in accordance withclaim 1 wherein the second material and the base are circular and thebase is rigidly held about its periphery and the deflection of saidsensor is defined by the following equation:

8. The condition sensor of claim 1 wherein said base member includes anormal projecting sidewall defining a cup shaped shell having aperipheralmounting means, and said second material is integrally bondedthroughout the interface to the outer surface of the base, the area ofthe second material being substantially less than said base and spacedfrom said sidewall.

9. The condition sensor of claim 8 wherein said base member is acup-shaped shell having a peripheral mounting flange adapted to beinterconnected with a member to define an enclosed chamber. I

10. In a bi-material humidity sensor in accordance with claim 1 whereinthe second material and the base are circular and the base isessentially rigidly held about its periphery and the sensitivity tohumidity of said sensor is defined by the following equation:

11. In a bi-material temperature sensor in accordance with claim Iwherein the second materialand the base are circular and the base isessentially rigidly held about its periphery and the sensitivity totemperature to said sensor is defined by the following equation:

12. In a bi'material condition sensor adapted to establish a deflectionin accordance with the sensed condition comprising a generally flatsensing element having an outer bi-material peripheral portion and abonded core, said peripheral portion having a high coefficient ofexpansion material to a first side and a relatively low coefficient ofexpansion material to'a second side, said'core having a low coefficientof expansion material to said first side and a relatively highcoefficient of expansion material to said second side.

13. The condition sensor of claim 12, wherein said element is a circularplate and the first and second high coefficient of expansion materialsare of the same material and have the same expansion coefficient valuesand the first and second low coefficient of expansion materials are ofthe same material and have the same expansion coefficient values. I

14. in a bi-material humidity and temperature sensor in ac cordance withclaim 1 wherein the second material and the base are circular and thebase is rigidly held about its periphery, said second material and saidbase including an outer bi'material peripheral portion and a bondedcore, said peripheral portion having a high coefficient of expansionmaterial to a first side and a relatively low coefficient of expansionmaterial to a second side, said core having a low coefficient ofexpansion material to said first side and a relatively high coefficientof expansion material to said second side and the deflection of saidsensor is defined by the following equation:

15. The condition sensor of claim 1 wherein said base member is acircular plate and said'mounting means is secured to the peripherythereof and said second material is ringshaped and secured .to the outerperiphery of the base member.

16. The condition sensor of claim 1 wherein said base member is acircular plate and said mounting means is secured to the peripherythereof and said second material is ringshaped and secured to the outerperiphery of the base member, and a third material having a coefficientcorresponding to said second material and applied as a central discmember to the opposite surface of said base member from said secondmaterial.

17. A bi-material humidity sensor adapted to establish a deflection inaccordance with the sensed condition comprising a flexible base memberhaving a predetermined humidity coefficient of expansion, said basemember being a substantially planar member,

fixed mounting means connected at opposite sides of said base member torigidly support said element at said opposite sides, and

a layer of cellulose acetate butyrate cross-linked withureaforrnaldehyde and directly bonded to said base member and having adifferent humidity coefficient of expansion, said layer being spacedfrom said mounting means and the periphery of said base member toestablish a free uncovered portion of said base member,

18. The condition sensor of claim .17 wherein said base member is analuminum plate-like member having the surface to which said secondmaterial is secured having a thin zinc plated layer covered by a thincopper plated layer, said copper layer being oxidized to improve theadhesion of the second layer.

19. In a bi-material sensor in accordance with claim 1 wherein saidsecond material is a reaction product of a cornpound containingglucoside chains and a stabilizing monomer or partial polymer capable ofcross-linking with the hydroxyl groups of said glucoside.

20. A bi-material condition sensor element adapted to be fixedly andrigidly mounted at a pair of oppositely located areas of the element andestablishing a deflection therebetween in accordance with the sensedcondition comprising a substantially planar base member having apredetermined coefiicient of expansion for said condition, and

a second material having a different coefficient of expansion for saidcondition and integrally bonded through the interface of the base memberand the second material to a selected portion of said base memberthroughout, said selected portion being substantially less than the areaof said base member to establish a free uncovered portion of said basemember.

21. The condition sensor of claim 20 wherein said base member is acircular plate adapted to be fixedly mounted about the periphery thereofand said second material is secured to the center of the base member.

22. The condition sensor of claim 20 wherein said base member is acircular plate adapted to be fixedly mounted about the periphery thereofand said second material is secured to the outer peripheral portionthereof.

23. The bi-material condition sensor of claim 1, wherein said fixedmounting means is connected essentially to the full periphery of saidbase member and said second material is symmetrically secured to thebase member.

- II l I! 5 UNITED STATES PATENT OFFICE cETiFicATE or CECTEN Patent No.3 665 ,765 Dated May 30 1972 Invent0r( )\/ICTOR J. DUBE. PAUL E. THOMAand JAO SI-IIUN KAO It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 3, Line 34, after "alloy" cancel "110-0" and insert llO O-OColumn 5, Line 8, after "1'' insert an oblique sign Column 5, Line 62,cancel (1 v) and insert Column 7, Line 52 at the beginning of the lineinsert M Column 7, Line 58, cancel the "equal sign second occurrence,and insert a minus sign Signed and sealed this 26th day of December1972.

(SEAL) Attest:

EDWARD I IJLE JPCHER,JR ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PO-105O (10-69) uscoMM-Dc scam-ps9 (1 11,5 GOVEFNMENTPRINTING OFFICE [969 0-456-334

2. The bi-material condition sensor of claim 1 wherein said fixed mounting means is connected essentially to the full periphery of said base member.
 3. The condition sensor of claim 1 wherein said second material is symmetrically secured to the base member.
 4. The bi-material condition sensor of claim 1 wherein said second material is integrally bonded throughout the interface to the central portion of said base member in substantially spaced relation to said fixed mounting means.
 5. A bi-material sensor in accordance with claim 4 wherein the base member is rigidly held about its periphery.
 6. The condition sensor of claim 1 wherein said base member is a circular plate and said mounting means is secured to the periphery thereof and said second material is secured to the center of the base member.
 7. In a bi-material humidity and temperature sensor in accordance with claim 1 wherein the second material and the base are circular and the base is rigidly held about its periphery and the deflection of said sensor is defined by the following equation:
 8. The condition sensor of claim 1 wherein said base member includes a normal projecting sidewall defining a cup-shaped shell having a peripheral mounting means, and said second material is integrally bonded throughout the interface to the outer surface of the base, the area of the second material being substantially less than said base and spaced from said sidewall.
 9. The condition sensor of claim 8 wherein said base member is a cup-shaped shell having a peripheral mounting flange adapted to be interconnected with a member to define an enclosed chamber.
 10. In a bi-material humidity sensor in accordance with claim 1 wherein the second material and the base are circular and the base is essentially rigidly held about its periphery and the sensitivity to humidity of said sensor is defined by the following equation:
 11. In a bi-material temperature sensor in accordance with claim 1 wherein the second material and the base are circular and the base is essentially rigidly held about its periphery and the sensitivity to temperature to said sensor is defined by the foLlowing equation:
 12. In a bi-material condition sensor adapted to establish a deflection in accordance with the sensed condition comprising a generally flat sensing element having an outer bi-material peripheral portion and a bonded core, said peripheral portion having a high coefficient of expansion material to a first side and a relatively low coefficient of expansion material to a second side, said core having a low coefficient of expansion material to said first side and a relatively high coefficient of expansion material to said second side.
 13. The condition sensor of claim 12, wherein said element is a circular plate and the first and second high coefficient of expansion materials are of the same material and have the same expansion coefficient values and the first and second low coefficient of expansion materials are of the same material and have the same expansion coefficient values.
 14. In a bi-material humidity and temperature sensor in accordance with claim 1 wherein the second material and the base are circular and the base is rigidly held about its periphery, said second material and said base including an outer bi-material peripheral portion and a bonded core, said peripheral portion having a high coefficient of expansion material to a first side and a relatively low coefficient of expansion material to a second side, said core having a low coefficient of expansion material to said first side and a relatively high coefficient of expansion material to said second side and the deflection of said sensor is defined by the following equation:
 15. The condition sensor of claim 1 wherein said base member is a circular plate and said mounting means is secured to the periphery thereof and said second material is ring-shaped and secured to the outer periphery of the base member.
 16. The condition sensor of claim 1 wherein said base member is a circular plate and said mounting means is secured to the periphery thereof and said second material is ring-shaped and secured to the outer periphery of the base member, and a third material having a coefficient corresponding to said second material and applied as a central disc member to the opposite surface of said base member from said second material.
 17. A bi-material humidity sensor adapted to establish a deflection in accordance with the sensed condition comprising a flexible base member having a predetermined humidity coefficient of expansion, said base member being a substantially planar member, fixed mounting means connected at opposite sides of said base member to rigidly support said element at said opposite sides, and a layer of cellulose acetate butyrate cross-linked with urea-formaldehyde and directly bonded to said base member and having a different humidity coefficient of expansion, said layer being spaced from said mounting means and the periphery of said base member to establish a free uncovered portion of said base member.
 18. The condition sensor of claim 17 wherein said base member is an aluminum plate-like member having the surface to which said second material is secured having a thin zinc plated layer covered by a thin copper plated layer, said copper layer being oxidized to improve the adhesion of the second layer.
 19. In a bi-material sensor in accordance with claim 1 wherein said second material is a reaction product of a compound containing glucoside chains and a stabilizing monomer or partial polymer capable of cross-linking with the hydroxyl groups of said glucoside.
 20. A bi-material condition sensor element adapted to be fixedly and rigidly mounted at a pair of oppositely located areas of the element and establishing a deflection therebetween in accordance with the sensed condition comprising a substantially planar base member having a predetermined coefficient of expansion for said condition, and a second material having a different coefficient of expansion for said condition and integrally bonded through the interFace of the base member and the second material to a selected portion of said base member throughout, said selected portion being substantially less than the area of said base member to establish a free uncovered portion of said base member.
 21. The condition sensor of claim 20 wherein said base member is a circular plate adapted to be fixedly mounted about the periphery thereof and said second material is secured to the center of the base member.
 22. The condition sensor of claim 20 wherein said base member is a circular plate adapted to be fixedly mounted about the periphery thereof and said second material is secured to the outer peripheral portion thereof.
 23. The bi-material condition sensor of claim 1, wherein said fixed mounting means is connected essentially to the full periphery of said base member and said second material is symmetrically secured to the base member. 